1. Field of the Invention
The present invention relates to an optical phase modulator and optical phase modulating device that are used in optical phase modulating using optical phase.
2. Description of the Related Art
In addition to conventional intensity modulation as a method of modulating light in optical communication by an electrical signal, there is also optical phase modulation. Optical phase modulation differs from conventional optical intensity modulation, which performs modulation between ON and OFF of the optical intensity; for example, the followings are known, i.e., there is binary optical phase modulation (for example, DPSK: Differential Phase Shift Keying) that performs modulation using two different optical phases, 4-value optical phase modulation (for example, DQPSK: Differential Quadrature Phase Shift Keying) that performs modulation using four different optical phases, or 16QAM (Quadrature Amplitude Modulation) that uses a combination of sixteen types of different amplitudes and phases). In optical phase modulation, generally it is possible to improve the receiving sensitivity when compared with optical intensity modulation. In addition, in multi-value optical phase modulation such as the DQPSK method and 16QAM method, it is possible to transmit a plurality of bits of information per one phase (symbol), so it is possible to efficiently increase the transmission capacity. In order to generate a phase modulation signal, a binary optical modulator (DPSK optical phase modulator) that uses a Mach-Zehnder modulator (MZ modulator) such as disclosed in patent literature 1 is used.
FIG. 16 is a schematic diagram illustrating the construction of a DPSK optical phase modulator that comprises a MZ modulator as an example of a binary optical phase modulator. The MZ modulator 11 comprises two optical waveguide path arms 11a, 11b. The light that enters into the MZ modulator is divided and guided into each of the optical waveguide path arms 11a, 11b. Electrodes 11a1, 11b1 are provided in the optical waveguide path arms 11a, 11b, respectively, and the phase of the light passing through each optical waveguide path arm 11a, 11b is changed by using the change in the index of refraction due to the electro-optical effect when voltage is applied to the electrodes. After that, these two lights are combined. Here, in the optical waveguide path arms 11a, 11b, the difference in the optical path lengths is adjusted so that the initial phase difference in the used wavelength when there is no modulation is π, or in other words, so that the output light is in a extinction state.
FIG. 17A is a diagram in which the phase of the optical field of the light that passes through the optical waveguide path arms 11a, 11b is expressed in polar coordinates using the in-phase component I and the quadrature component Q. In FIG. 17A, the distance from the origin expresses the amplitude of the electric field, and the angle from the I axis expresses the phase angle. The voltage V11a that is applied to the upper optical waveguide path arm 11a via the electrode 11a1 changes around the reverse bias voltage (V11a=−Vb) between −Vb+ΔV to −Vb−ΔV (where Vb>ΔV), and the phase of the output optical field P11a of the upper optical waveguide path arm 11a changes between π and 0 according to this change. Moreover, the voltage V11b that is applied to the lower optical waveguide path arm 11b via the electrode 11b1 changes around the reverse bias voltage (V11b=−Vb) between −Vb−ΔV to −Vb+ΔV, and the phase of the output optical field P11b of the lower optical waveguide path arm 11b changes between −π and 0 according to this change. Therefore, when a signal ΔV and an inverted signal of this signal are applied to the optical waveguide path arms 11a, 11b and the reverse bias voltage (−Vb) is superimposed on this, the phase of the output optical field P11 from the MZ modulator, in which light from each optical waveguide path arm 11a, 11b is combined, becomes 0 (V11a=−Vb−ΔV, V11b=−Vb+ΔV) or π(V11a=−Vb+ΔV, V11b=−Vb−ΔV). In other words, in the MZ modulator 11, phase modulation of the output optical field P11 is performed on the I axis between the two values 0 and π. FIG. 17B is a diagram illustrating the relationship between the voltage (−V11a, −V11b) that is applied to the optical waveguide path arms 11a, 11b and the phase of the output light.
FIG. 18 is a schematic diagram illustrating the construction of a DQPSK optical phase modulator as one example of a 4-value optical phase modulator. As illustrated in FIG. 18, in the DQPSK optical phase modulator 13 two of the MZ modulators (DPSK optical phase modulators) illustrated in FIG. 16 (11-1, 11-2) are connected in parallel using a branching optical waveguide path 31 and combining optical waveguide path 32, where a phase shifter 30 that shifts the phase of the passing light by just π/2 is located on one of the branches of the branching optical waveguide path 31. FIG. 19 A and FIG. 19B express the phase of the output light from the DPSK optical phase modulators 11-1, 11-2 in polar coordinates using the I component and Q component, where in DPSK optical phase modulator 11-1 phase modulation is performed on the I axis between the two phase values 0 and π as described above, and in the DPSK optical phase modulator 11-2, the π/2 phase shifter 30 shifts the phase by just π/2, so phase modulation is performed on the Q axis between the two values π/2 and 3π/2. As a result, the phase of the output light from the DQPSK optical phase modulator 13 becomes the phase of the sum of the vectors of the output electric fields P11-1, P11-2 that are output from the two DPSK optical phase modulators 11-1, 11-2, and as illustrated in FIG. 18C becomes π/4, 3π/4, 5π/4 and 7π/4 according to the signal voltage that is applied to the DPSK optical phase modulators 11-1, 11-2.
As the number of binary optical phase modulator arranged in parallel further increases, it is possible to achieve even higher multi-value phase modulation (quadrature amplitude modulation) such as 16QAM or 256QAM. A binary optical phase modulator outputs light having the two phases 0 or π according to the input signals in this way, and multi-value optical phase modulators such as a 4-value optical phase modulator or 16QAM can be constructed by combining a plurality of binary optical phase modulators.
MZ modulators that use LiNbO3 as the material have already been put into practical use. A MZ modulator that uses LiNbO3 takes advantage of the Pockets Effect as an electro-optical effect. A MZ modulator that uses LiNbO3 is characterized by having small light loss, however, the size is several inches, which is large, and the driving voltage is also high. On the other hand, a MZ modulator that uses a semiconductor such as InP uses the Franz-Keldysh Effect or Quantum Confined Stark Effect (QCSE) as an electro-optical effect, so the modulator can be made more compact than a modulator that uses LiNbO3, and has a lower driving voltage. In addition, this MZ modulator has the advantage in that integration with devices such as a light source is easy. Therefore, in the future, in the case where construction of a modulator is complicated such as a multi-value optical phase modulator, a semiconductor MZ modulator, which is capable of miniaturization and integration, is promising.    Patent Literature 1: Japanese Patent Application No. H9-061766
However, a conventional semiconductor MZ modulator has the following problems. That is, in a semiconductor MZ modulator, as was described above, the Franz-Keldysh Effect or Quantum Confined Stark Effect is used, so as voltage is applied, not only does the optical phase change, but the absorption coefficient changes as well. Therefore, as illustrated in FIG. 20, when a voltage in the reverse direction is applied, damping of the optical field becomes large, so the amplitude of that optical field becomes small. As a result, an orthogonal component occurs in the optical field P11 of the output light that is output as a sum of vectors of the optical fields P11a, P11b of the optical waveguide path arms 11a, 11b. For example, when expressing phase 0, the voltage applied in the reverse direction to the optical waveguide path arm 11a is large (V11a=−Vb−ΔV), and the voltage applied in the reverse direction to the optical waveguide path arm 11b is small (V11b=−Vb+ΔV). Therefore, absorption of light by the optical waveguide path arm 11a is greater than the absorption of light by the optical waveguide path arm 11b, and vertical asymmetry occurs in the amplitude of the optical field as illustrated in FIG. 20. Consequently, when the voltage that is applied to the optical waveguide path arms 11a, 11b is changed between −Vb−ΔV and −Vb+ΔV, an orthogonal component (Q component) occurs in the trajectory of the vector sum of the optical fields as illustrated in FIG. 20. The phase difference that is due to this orthogonal electric field becomes the cause of frequency chirping, and when transmitting an optical signal, the transmitted signal degrades due to fiber dispersion.
Taking into consideration the problems described above, the object of the present invention is to provide an optical phase modulator and optical phase modulating device that are capable of reducing the effect of frequency chirping due to an orthogonal component that is caused by absorption of light by a semiconductor.